Many home and personal care formulations seek to deliver so-called benefit agents to substrates such as cloth, hair and skin. Encapsulation of the benefit agent in particles has been proposed as a means of enhancing delivery, which is advantageous because of the expense of some benefit agents. Delivery of particles per se can also be useful where the particles, even in the absence of specific benefit agents, confer a benefit.
These particles may comprise polymers and many different types of polymerisation are known. In the present specification a distinction will be drawn between step-growth and chain-growth polymerisation. This is the well-established reaction mechanism distinction drawn by Paul Flory in 1953 (see Paul J. Flory, “Principles of Polymer Chemistry”, Cornell University Press, 1953, p. 39. ISBN 0801401348).
For the purposes of the present specification a chain-growth polymer is a polymer which is formed by a reaction in which monomers bond together via rearrangement (for example, of unsaturated and typically vinyllic bonds, or by a ring-opening reaction) without the loss of any atom or molecule. Chain-growth polymers grow in a single direction from one end of the chain only and an initiator is typically used. In chain-growth polymerisation it is commonplace that once a growth at a chain end is terminated the end becomes unreactive.
An example of one type of chain-growth polymerisation is the free-radical polymerisation reaction, for example the well-known polymerization of styrene (vinyl benzene) in the presence of benzoyl peroxide (as radical initiator) to produce polystyrene. Similarly, aluminum chloride may be used to initiate the polymerisation of isobutylene to form synthetic rubber. Other examples include the polymerization reactions of acrylates or methacryates.
A step-growth polymer is a polymer whose chain is formed during by the reaction of poly-functional monomers to form increasingly larger oligomers. Growth occurs throughout the matrix and the monomer level falls rapidly in the early stages of the reaction. No initiator is needed for a step growth polymerisation and the ends of the growing chain generally remain active at all times. Typically (but not always) a small molecule, which is often water, is eliminated in the polymerization process.
An example of step-growth polymerization is the formation of polyester by the reaction of dicarboxylic acids and glycols with elimination of water. Another example is the polymerisation of phenol and formaldehyde to produce “Bakelite”. Other well known step-growth polymerisation reactions are the formation of polyesters, polyurethanes, polyureas, polyamides and polyethers.
It should be noted that chain-growth polymerisation and so-called “addition polymerisation” are different concepts. Addition polymerisation is where the reaction product is a polymer only. This may be contrasted with “condensation polymerisation” where a small molecule (the “condensate”) is also produced. Polyurethane, for example, is produced by addition polymerisation of (di)isocyanate compounds (R—N═C═O) with (di)hydroxy compounds (HO—R) to form the urethane/carbamate linkage (R—NH—CO—O—R), but the reaction mechanism is step-growth rather than chain-growth as there is molecular rearrangement without elimination of a small molecule.
Both chain-growth and step-growth have been used to prepare particles by polymerisation in which some of the components are present in the dispersed phase of an emulsion. In the case of chain-growth, all of the components may be present in droplets of the dispersed phase which, once initiated, react internally to form a particle. In the case of step-growth, components may be present both in the dispersed and the continuous phase to react at the dispersed phase surface to form a “shell” at the interface.
In US 2009/312222 particles are prepared using so-called “mini-emulsion” polymerisation, to give a particle with a size as from about 30 to 500 nm. The polymer comprises units derived from monomers that are capable of undergoing chain-growth free-radical polymerisation. GB 2432851 discloses particles derived from monomers that are capable of undergoing free-radical polymerisation. GB 2432850 discloses core/shell particles in which both the core and the shell comprises monomer units which are derived from monomers that are capable of undergoing free-radical polymerisation.
Emulsion polymerisation can also be performed using step-growth reactions. U.S. Pat. No. 4,622,267 discloses an interfacial polymerization technique for preparation of microcapsules. US 2002/169233 discloses an interfacial polymerization process wherein a microcapsule wall of a polyamide, an epoxy resin, a polyurethane, a polyurea or the like is formed at an interface between two phases. The core material is initially dissolved in a solvent and an aliphatic diisocyanate soluble in the solvent mixture is added. Subsequently, a non-solvent for the aliphatic diisocyanate is added until the turbidity point is just barely reached. This organic phase is then emulsified in an aqueous solution, and a reactive amine is added to the aqueous phase. The amine diffuses to the interface, where it reacts with the diisocyanate to form polymeric polyurea shells.
Microcapsules have been proposed in which the wall material comprises both a step-growth polymer and a chain-growth polymer.
US 2005/0153839 disclose microcapsules for use in the production of multicolour thermo-sensitive recording materials having polyurethane or polyurea walls. The polymer wall includes (via a covalent bond) a polymer obtained by radically polymerising at least a vinyl monomer further comprising a polyether. Preferably the raw materials for the walls are di-isocyanates. It should be noted that the vinyl polymer is included in the wall rather than being enclosed by it.
EP 2204155 discloses leak-proof, friable core-shell fragrance microcapsules which have melamine-formaldehyde (step-growth polymer) shells and in which the core may optionally comprise, among other possibilities, high density organic oil-soluble ingredients which may be prepared by any standard means such as radical polymerisation of unsaturated monomers such as vinyl or acrylic monomers (which are chain-growth polymers). Alternatively the polymers may be prepared by condensation reactions such as those leading to polyethers or polyesters (which are step-growth polymers). The fragrance comprises at least one cyclic fragrance material. The reason for including these pre-formed high density materials is to match the density of the micro-capsules with that of the composition in which they are used, to prevent separation.
An effective encapsulate for a benefit agent, for example a benefit agent such as perfume, should have the following properties:                It should have a target loading of 20% w/w benefit agent or better and be easy to load;        It should minimise leakage of the benefit agent into a product during manufacture and on storage;        It should not require modification of the bulk formulation, for example by requiring the presence of structuring and/or suspending systems;        Ideally, the encapsulate should deposit well onto substrates;        The encapsulate should control the release of benefit agent.        